Accurate to the Inch

Real-Time Kinematic (RTK) base station networks are providing an increasing number of farmers with an affordable way to access the most accurate global positioning system (GPS) signal available. The networks provide GPS information that is accurate to within 1 in. of a farmer's actual location.

In the past, farmers who wanted that level of accuracy would have to buy a second GPS receiver and radio and install them in a fixed, nearby base station. This equipment could cost anywhere from $15,000 to $20,000. But now companies are putting up networks of base stations and charging farmers an annual fee to use the signals.

“One of the things we do is give farmers opportunity to rent space, like you would buy cell-phone minutes,” says Steve Cubbage, owner of Record Harvest Enterprises, a precision farming company and Trimble dealer. “They can rent a year subscription for $1,500, whereas if they had to buy that base station for themselves it would be $12,000 or more. This does not include the typical $22,000 to $25,000 price tag for the automated steering system on the tractor.”

What is RTK?

RTK signal correction has been commercially available for almost a decade, but it hasn't been affordable until recently. According to Trimble Ag, the technology requires two specialized GPS receivers and two radios. One set is used on the farm implement and the other is set up within six miles in a known location to act as a reference point.

The reference or base station receiver calculates for error and sends the corrected position to the receiver and radio on the tractor. Both receivers collect data from a GPS satellite L2 Band carrier phase signal in addition to standard L1 Band signals to allow for better precision.

“I call RTK the Holy Grail in the autosteer market because it gives farmers 1-in. pass-to-pass and year-to-year repeatable accuracy,” Cubbage says. Other less precise types of correction signals are accurate from 2 to 8 in. on a pass-to-pass basis, or for the 15 minutes it takes to make one field pass. But accuracy drops to as much as 3 ft. after 15 minutes as new satellites come over the horizon.

Despite the accuracy advantage of RTK, few farmers adopted it because of the high cost of the base station. “That's when Trimble started to promote the idea of dealer-owned RTK network stations where customers could just pay a fee to access the RTK signal, much like signing up for cellular service,” Cubbage says.

Networks form

Trimble Ag formed its first network in 2001. Since then, dozens of networks have been formed by farmers, implement dealers, and precision agriculture equipment dealers, including AGCO, Case IH, John Deere, AutoFarm and Record Harvest.

Today networks are growing phenomenally fast. Between 2005 and 2006, the number of acres covered more than doubled, according to Trimble. The company estimates that RTK base station networks now cover about 120 million acres of U.S. farmland. Trimble, which provides coverage for Case IH and New Holland dealers, claims about 75% of those acres.

“RTK networks are becoming a very big item in the Midwest,” says Levi Kettle, AutoPilot sales channel manager with Trimble. “More and more farmers, including some who haven't yet purchased automated steering systems, are investing in networks under construction in their area.”

Kettle says networks allow farmers to gain the benefit of sub-inch GPS accuracy and performance without having to spend the money to install their own, on-farm RTK system or expend the time and energy required to operate and maintain such systems.

“Customers can just start their tractors, power up the system, go to the field, and start working,” he says. “That is hitting what ag customers want from the technology as opposed to having to worry about equipment outside the tractor.”

In return, dealerships that provide network service gain an additional revenue stream and encourage the sale of RTK-controlled automated steering systems by lowering the total cost by about 35%.

AutoFarm, which claims to be the first company to sell RTK automatic steering systems in the United States, also has seen the growth in networks. “They are growing fairly quickly,” says Travis Murdock, AutoFarm's corporate communications director. “Because of our early start, we have many large RTK networks.” One of the largest dealer-owned AutoFarm networks is operated by Precision Ag Equipment in Illinois. The network includes more than 70 base stations and is growing.

“In our business, we say accuracy is addictive,” Murdock says. “If your neighbor has RTK and you see that they are getting good results, then you want to try it, too. A network lets everyone get the same results.”

Equipment required

Murdock explains that an RTK system can be divided into two components: the equipment that goes on the tractor, called roving components, and the base station, which is fixed.

By joining a network, the only components you need are the rovers, a shorthand word for the antenna and receiver that go on the tractor, and the radio that communicates with the base station. Those components must be capable of receiving RTK signal correction. Costs range from $2,000 to $4,000 for the roving receiver and are about $2,000 for the radio.

Should you subscribe?

Network providers say that, before you consider an RTK system, you should first ask whether any of your operations require sub-inch accuracy that is repeatable year to year.

“If you are not coming back to do any secondary operations such as cultivating or sidedressing after you've planted, then you don't need that sub-inch repeatable accuracy,” says Wade Stewart, AGCO area business manager. Examples include yield monitoring or tagging information during scouting.

However, if you do plan to return to the field at a later date and need to get to the same run lines or area in the field within an inch, then RTK is the better option. Such cases include strip tillage, controlled traffic, precision fertilizer and chemical application, and laying drip irrigation tape.

Once you decide on RTK signal correction, you need to decide whether to subscribe to a network or buy your own base station. Matt Danner, senior product support specialist at John Deere Ag Management Solutions (AMS), says if your farm is contained within the six-mile coverage range of one base station, then it may be advantageous to own your own. “That way you are not at anyone's liberty for losing power, and you are in full control of managing the station,” Danner says. “But if you are past six miles, which a lot of farms are, you will get more benefit by joining a network because you will get more coverage at a fixed cost.”

Another factor is the number of vehicles that will be using the signal at the same time. Some networks charge by the vehicle. In those cases, buying your own base station may be cheaper if you are running multiple vehicles.

Contact your dealer

If you are interested in subscribing to a network, you should contact your local farm machinery or GPS dealer to find out if a network is available in your area. “They are our first line of support,” says Deere's Danner. “If a couple of farmers are interested, a dealer will help out, either through support, owning or leasing the signal.”

Equipment companies often will provide the hardware but enlist the help of local GPS providers to put up networks. For example, AGCO has a list of company-approved providers that can coordinate base station network placement. “Local providers have already done the research and will know if space is available on existing towers or where a new tower should be located,” says AGCO's Stewart.

Fees and conditions of each network will vary based on the dealer. Some dealers waive the annual subscription fee and charge only a one-time configuration cost. For example, Illinois-based Precision Ag Equipment, an AutoFarm dealer, charges a one-time fee of about $3,500 to join the network.

“Suppliers of RTK correction services offer vastly different levels of quality, service and price,” says Trimble's Kettle. “So it pays to shop around.”

Buying tips

ROB LINDORES, director of marketing for Trimble Ag, says to weigh these factors when shopping for a supplier:

Signal reliability and accuracy

Ask potential service providers for names and phone numbers of current customers in your service area. Then ask the customers the following questions: How reliable is the RTK correction signal? Does the base station network provide continuous, accurate RTK signal correction 24 hours a day, 365 days a year? How accurate is the signal when the roving receiver is operating from two to six miles from the nearest base station? Look for a GPS system that delivers ±1-in. accuracy and repeatability up to six miles from the nearest base station.

Cost of service

“No supplier can afford to provide RTK network correction signals as a public service,” Lindores says. Suppliers who offer “free” RTK network services may be charging in other ways such as extra up-front charges for the automated steering system itself.

System operating life

Ask the suppliers, How soon before my RTK automated steering system or base station network will be obsolete? Will the equipment be able to support signals launched by future GPS satellites being planned by the U.S. Department of Defense? Will my equipment work with next-generation RTK base station networks?